In packaging applications, it can sometimes be desirable to seal a film package without the use of a separate adhesive film layer on the package. Such a sealing process can be obtained by using materials that can be self-adhered, or materials that can be made to adhere to or to adhere to another material. One such process is a process by which radio frequency (RF) energy is applied to a suitable material, and after absorption of the RF energy the material can subsequently be self-adhered, or adhered to different substrates to form a seal. This process is known as RF welding, and suitable materials for such a process are known in commercial practice. Polyvinyl chloride (PVC), for example, is a material that can be susceptible to RF energy and be very suitable for RF welding. PVC, however, is not an acceptable material for widespread use in some areas of the world and alternate materials are needed to perform the function of PVC in those regions where PVC is not desirable.
Selecting suitable alternate materials for use in packaging applications that use the RF welding technique is not a trivial exercise. In selecting or developing suitable alternative materials, at least three (3) criteria need to be met. First, the material must provide good physical and/or mechanical characteristics that are satisfactory for a package or a component thereof. Second, a suitable polymer film should be susceptible, that is should absorb, RF energy. Third, the polymer film should be capable of forming a bond, weld, or seal, when subjected to RF energy under the proper conditions of pressure and time.
Various polymers suitable for use in packaging applications are well known. For example, polyolefins such as polyethylene and polypropylene are useful in packaging. Some polymeric materials are known to be susceptible to RF energy. However, other materials may not be suitable for use in the applications described herein due to prohibitive cost or other problems. For example, some materials can be susceptible to RF energy but are not capable of forming a seal, or weld. Some materials can require more stringent process conditions, or require that a component be increased to the detriment of other physical/mechanical properties. The susceptibility of EVA, for example, is highly dependent on the vinyl acetate content. Typically ethylene (meth)acrylic acid copolymers and their ionomers are not RF susceptible.
Mixtures of ionomers and polar solvents such as glycerol can produce compositions that absorb RF energy. However, use of solvents having considerable vapor pressure at extrusion temperatures can lead to other problems, such as fogging during processing. Also, use of polar solvents can be undesirable for environmental reasons.
It can be desirable to have a composition that absorbs RF energy, and can be sealed using RF welding, while yet having physical and mechanical properties suitable for use in packaging applications.